Peroxide crosslinked polyethylene (PEX-a) pipes are prepared by soaking polyethylene (PE) material in contact with peroxides under an adequate period of time, often at elevated temperatures. For the formation of a three dimensional network to occur, where crosslinks tie the crystalline areas together in coexistence with a certain quantity of non-crosslinked molecules, heat is required to bring about the crosslinking. Subsequently, the PEX-a material may be processed to pipes in, e g an Engel extruder (ram extruder). The polyethylene material has to be well penetrated by peroxides prior to processing. The penetration of peroxides is enhanced by smaller particle sizes of the polyethylene since then the penetration of the peroxide into the voids of the particles will proceed more rapidly. Hence, polyethylene in powder form will be penetrated in a shorter time than PE pellets of standard sizes.
At present, polyethylene is supplied for peroxide crosslinked polyethylene, PEX-a, production as either powder, having an average particle size of 0.9 mm or pellets, with a particle size of 55-65 pellets/g. For the production of uncoloured PEX-a, polyethylene powder is used since PE is uncoloured, i e has a natural colour, by nature. However, the handling of polyethylene in powder form is associated with a number of drawbacks. For instance, it is not possible to manufacture coloured PEX-a by using powder PE since it is extremely difficult, if not impossible, to successfully mix colour pigments with powder PE. Other additives, such as antioxidants, may be dry mixed into the powder before processing. However, dry mixing involves problems with dust and the risks for dust explosions.
Another disadvantage involved with powder handling is the relatively low bulk density of powder, which is approximately 0.45-0.5 kg/m3, resulting in large volumetric volumes to transport and store. Also the sometimes poor dry flow properties of powder causes problems with the handling of the powder both for polymer producers as well as for pipe producers.
In order to manufacture coloured PEX-a pipes, PE pellets are used. This is because it is necessary to compound the colour pigments into the polyethylene material before processing. Pellets presently used for PEX-a production are relatively big pellets with a number of pellets per gram of approximately 20-30 pellets/g and up to 65 pellets/g, having a diameter of about 3-5 mm and a length of about 3-4 mm. As mentioned above pellets of large sizes result in extensive soaking times and often combined with high soaking temperatures. Normally, the pellet size distribution may involve some very big pellets which consequently lead to even longer soaking times, since the soaking time is proportional to the square of the distance from the particle surface to the centre of the particle.
In WO 00/35646 a method of producing pelletized polyolefin for rotational moulding is disclosed, having a particle size distribution of 0.1 to 1 mm, preferably 0.2 to 0.9 mm and a low water content of less than 0.1 weight %. These very dry pellets may be produced by mixing, pelletization and drying.